Meshes added depend on the viewport and are not placed on their default place
Open the tools panel (Press the T key) to have access to the creation parameters (or press F6 after adding your mesh). Make sure that the Align to view box is disabled.



If the question is about why is the mesh not created at the center of the grid, then the answer is that objects are created where the 3D cursor is on the scene.



To reset the 3D cursor to the center of the grid press Shift+C